The present invention relates to cementitious compositions embodying phase change materials and, more particularly, to cementitious compositions incorporating polyethylene glycols and/or end-capped polyethylene glycols as phase change materials. These compositions are useful in the manufacture of building materials.
There has been a great deal of interest in phase change thermal energy storage systems due to their inherent ability to store large amounts of heat and release it to the surrounding environment as temperatures drop below a predetermined level. These systems are of particular interest in the architectural and building trades where climate control and its concommitant energy consumption is one of the principal considerations in building design and material selection.
A variety of building materials and techniques have previously been used to conserve heat or cool and thereby reduce energy costs. Included among them are structural elements which incorporate phase change materials. By incorporating phase change materials into building materials, energy in excess of that necessary to maintain comfort conditions is inherently absorbed and released when the surrounding environment drops below the comfort range. Thus, in winter months, phase change materials incorporated into structural elements in the walls or floors of buildings and the like can absorb solar energy during daytime hours and release it to the interior at night as temperatures drop. In summer months, the same phase change material, due to its thermostatic character, conserves coolness by absorbing energy.
Structural elements incorporating phase change materials are more desirable than elements which store only sensible heat because they have a higher capacity to store energy and they absorb and release a large quantum of energy over a very narrow temperature range. A phase change material utilizes its latent heat of fusion for thermal storage. The latent heat of fusion is substantially greater than the sensible heat capacity of the material. That is, the amount of energy a material absorbs upon melting, or releases upon freezing, is much greater than the amount of energy it absorbs or releases upon increasing or decreasing in temperature 1.degree. C. Thus, upon melting and freezing, per unit weight, a phase change material absorbs and releases substantially more energy than a sensible heat storage material which is heated or cooled through the same temperature range. Furthermore, as contrasted with a sensible heat storage material which absorbs and releases energy essentially uniformly over a broad temperature range, a phase change material absorbs and releases a large quantum of energy in the vicinity of its melting/freezing point. This is particularly advantageous in buildings where space is at a premium and energy storage and release are required within a very narrow comfort range.
Among the teachings which were available in the art prior to the present invention are those of U.S. Pat. No. 4,259,401 to Chahroudi et al which discloses both structural and non-structural building materials incorporating phase change materials. These building materials are made up of a rigid porous matrix structure which is impregnated with the phase change material and sealed for waterproofness. Three classes of phase change materials are disclosed, namely, hydrated salts, waxes, and clathrates. Cements, plasters or thermosetting materials may form the rigid matrix. Both pre-formed structural elements such as building blocks, ceiling tiles, and dry wall and poured elements such as walls, floors and partitions are described in the patent. The encapsulation of phase change materials which interfere with set is also disclosed in the patent.
U.S. Pat. No. 4,277,357 to Boardman discloses a thermal energy storage composition similar to some of those described by Chahroudi et al wherein a salt hydrate is incorporated into a hydrated hydraulic cement.
U.S. Pat. No. 4,111,189 to Dizon discloses a combined solar collector and thermal energy storage device in which the collector employs phase change matrix comprising a phase change material dispersed in a polymeric matrix. The preferred phase change material is polyethylene glycol having a molecular weight in the range of 4500 to 20,000.